The invention relates to a process for making a crystalline solid-solution powder with a low electrical resistance by reacting at least two reactants at high temperatures and then rapidly cooling the reaction product in a high speed gas stream. The reaction is accomplished in a plasma arc of a plasma chamber provided with an inlet opening for the reactants and an outlet opening with a gas-supply unit to obtain a molten material, wherein a molten metal or alloy is used as the first reactant and a gas as the second reactant. The invention also relates to a crystalline solid-solution powder made thereby, especially an indium-tin-oxide crystalline solid-solution powder and the use thereof, especially as an ITO sputtering or vacuum coating target for preparing low electrical resistance thin films.
Several processes for making crystalline solid-solution powders are known. In numerous publications, special interest is focussed on indium-tin-oxide (ITO) powders, which are used for ITO sputtering targets, and on the production thereof. In this connection, high electrical conductivity of the target is regarded as a desirable feature because it allows achieving high sputtering rates with ITO targets. The high electrical conductivity of the target is substantially determined by the electrical resistivity of the ITO powder used for making the target by means of sintering. Similarly, a highly conductive coating formed using an ITO paste needs an ITO powder with correspondingly low electrical resistance.
EP 386 932 A1 discloses a process for making an ITO powder by thermal decomposition of concentrated mixed salts of indium and tin acetates. By means of this complex process, ITO powders having special properties relative to their particle characteristics are obtained. Such powders are further processed to targets with which ITO films can be obtained in vacuum-coating (sputtering) systems. EP 386 932 also provides data on the electrical conductivity of these ITO films, but not on the electrical conductivity or electrical resistance of the ITO powder or on the process parameters which influence this property.
Another chemical process for making ITO powders is by precipitating and then calcining indium hydroxide from an indium nitrate solution and tin hydroxide or hydrated tin oxide from an Sn chloride solution. However, quantitative data on the electrical resistivity of these or the above-mentioned ITO powders are generally not available. The resistivity of powders produced by calcining salts is generally found to be unsatisfactorily high. Similarly little information is provided on relationships between the electrical resistivity of the crystalline solid-solution powders and individual process features or parameters characterizing the processes used for making such powders.
The object of the invention is therefore to provide a process for making a crystalline solid-solution powder, especially indiumtin-oxide powder, which has low electrical resistivity, lower than 100 .OMEGA.cm, wherein the process does not include any wet-chemical process steps. The crystalline solid-solution powder obtained in this way is suitable for further processing in powder metallurgy processes without further treatments, such as annealing under reducing atmosphere. Another object is to provide a vacuum coating or sputtering target of low electrical resistivity which can impart the low resistivity properties to a thin film and to provide such a low resistivity (high conductivity film).